This invention relates to methods and apparatus for multipole acoustic logging, for example for logging the shear wave propagation characteristics of earth formations traversed by a borehole.
Conventional acoustic logging of earth formations traversed by a borehole is accomplished by means of a logging tool lowerered into the borehole on an armored communication cable. Such a tool typically incorporates several acoustic transducers, at least one being operated to produce acoustic signals and one or more others being operated to detect such signals. These transducers are commonly made of piezoelectric ceramic or magnetostrictive materials which expand and contract transversely to their surfaces (i.e. change in thickness) in response to electrical excitation, or conversely generate electrical voltages between those surfaces when subjected to pressure fluctations. Thus the transmitter can be driven with an appropriate oscillating electrical signal to generate pressure fluctuations in liquid in the borehole. These pressure fluctuations propagate as acoustic signals through the liquid and into and through the surrounding formations. The signals traversing the formations couple back into the borehole liquid, for example in the vicinity of the receivers, causing pressure fluctuations which result in electrical voltages at the outputs of the receivers. These voltages are sensed and amplified, and may be processed downhole to extract information for transmission up the cable; alternatively the waveforms of the received signals may be transmitted uphole, for example as digitized time samples, for processing at the surface. The excitation of the transmitter is commonly pulsed, enabling determination for example of the time taken by an acoustic signal to propagate the known distance between the transmitter and the receiver, and thus of the velocity (or its inverse, slowness) of acoustic propagation in the formations.
Although the type of transmitter most commonly used (a cylinder) generates pressure or P waves, known tools are not limited to investigating the propagation of such waves. When the P wave reaches the borehole wall, some of the acoustic energy is typically converted into other modes of acoustic propagation. Thus, shear or S waves may be excited in the formation, and Stoneley waves (including the low-frequency type known as tube waves) may be excited at the borehole/formation interface. Because these different modes generally travel at different speeds these additional modes can sometimes be distinguished in the receiver signals. Determination of such parameters as the speed and attenuation of P, S and Stoneley waves is useful in investigating a variety of subsurface formation properties of interest in the exploration for hydrocarbons and other valuable raw materials.
However, in certain circumstances transmitters which generate pressure waves are not effective in inducing propagation of S waves in a manner that permits their detection from within the borehole. If the speed of S waves in the formation is less than the speed of acoustic (pressure) waves in the borehole liquid, as in the case of a so-called `soft` or `slow` formation, shear wave energy in the formation cannot induce corresponding phenomena in the borehole liquid so the shear wave cannot be detected and its velocity cannot be found.
Various techniques have been suggested to facilitate the logging of shear wave properties of subsurface formations irrespective of relative borehole and formation sonic properties. Thus, a variety of sources for exciting shear waves directly have been proposed. These have included various forms of contact devices (U.S. Pat. Nos. 3,354,983, 3,683,326, 4,380,806, 4,394,754, 4,549,630). U.S. Pat. No. 3,475,722 teaches the use of three geophones arranged along mutually orthogonal axes on a common mount pressed into contact with the borehole wall. However all these devices suffer from the problem of requiring direct contact with the formation, and the consequent difficulties of limitation on logging speed, risk of the tool sticking and bad contact in poorly consolidated formations.
More recently, various kinds of multipole or azimuthally asymmetric transducers which can be suspended in the borehole liquid have been proposed for direct or indirect shear wave logging, such as dipole transducers (U.S. Pat. Nos. 3,593,255, 4,207,961, 4,383,591, 4,516,228; GB patent specification 2,124,377; EP patent specification 0,031,898), quadrupole transducers (GB patent specifications 2,122,351, 2,132,763) and octopole transducers (GB patent specification 2,130,725). Another suggestion has been to use geophones suspended in the liquid in a borehole and with a buoyancy adjusted to be nearly neutral for sympathetic movement with the borehole wall (U.S. Pat. No. 4,369,506); one such proposal (U.S. Pat. No. 4,542,487) provides geophones in orthogonally mounted pairs. These buoyancy adjustments are difficult to make accurately and significantly complicate the use of such devices.
Furthermore, previously known devices have relied on the use of non-rigid sondes, for example using cable or rubber hose to interconnect portions of the logging apparatus, in order to reduce problems introduced by propagation of extensionsal and bending acoustic energy directly along the sonde from the source to the detector. Such non-rigid sondes lack ruggedness and are therefore neither easy to use nor able to withstand prolonged operation in the rigorous environment typical of borehole logging.
It is an object of this invention to provide a method and apparatus for shear wave acoustic logging of subsurface earth formations which does not require direct physical contact between the logging apparatus and the formation.
It is also an object of this invention to provide a method and apparatus for shear wave acoustic logging of subsurface earth formations which provides information as a function of azimuthal direction around the tool.
A further object of the invention is to provide a method and apparatus for shear wave acoustic logging of subsurface earth formations which enables quantification of formation or rock anisotropy and identification of minimum and maximum stress directions. Such information is valuable in the interpretation of seismic records, the planning of enhanced oil recovery and the planning of hydrofracturing operations for example.